Question

I’m looking to a scatter plot with a rotated X axis. Basically, I want to plot correlations between 2 Y-axes. Ideally, I’d like to have the x-axis represent the time and Y-axes represent the correlations

data <- data.frame( words = c( "Aliens", "Aliens", "Constitution", "Constitution",    "Entitled", "Entitled" ),
              dates =  as.Date( c ("2010-01-05", "2010-02-13", "2010-04-20", "2010-06-11","2010-03-18", "2010-09-13" )), 
                    Rep =    c( .18, .14, .16, .45, .33, .71 ), Dem = c( .16, .38, .24, .11, .59, .34 ))

And this is what I was able to do so far. I don’t think it really gets the point across. I could size by correlation and color by month?

plot(x=data$dates, y=data$Rep, ylim=c(0,1.1*max(data$Rep)),
 col='blue', pch = 15,
 main='Rep Correlations stock close', xlab='date', ylab='Republican')
axis(2, pretty(c(0, 1.1*max(data$Rep))), col='blue')
par(new=T)
plot(x=data$date, y=data$Dem, ylim=c(0,1.1*max(data$Dem)),
 col='green', pch = 20,
 xaxt='n', axes = F, xlab = '', ylab='')
axis(4, pretty(c(0, 1.1*max(data$Dem))), col='green')
mtext("Democrat",side=4)

Any thoughts/tips?

Answer 1

Following up on @JohnColby’s comment above (and see How can I plot with 2 different y-axes? , http://rwiki.sciviews.org/doku.php?id=tips:graphics-base:2yaxes for arguments why you should not create dual y-axis plots if you can help it ), how about:

dat <- data ## best not to use reserved words -- it can cause confusion
library(ggplot2)
theme_update(theme_bw())  ## I prefer this theme
## code months as a factor
dat$month <- factor(months(dat$dates),levels=month.name)
dat <- dat[order(dat$dates),]
qplot(Rep,Dem,colour=month,data=dat)+
    geom_path(aes(group=1),colour="gray")+geom_point(alpha=0.4)+
    geom_text(aes(label=words),size=4)

(adding lines between the points, then re-plotting the points so they’re not obscured by the line; adding the words is cute but might be too much clutter for the full data set)

Or encode date as a continuous variable

ggplot(dat,aes(Rep,Dem,colour=dates))+
    geom_path(aes(group=1),colour="gray")+geom_point(alpha=0.4)+
    geom_text(aes(label=words),size=4)+
    expand_limits(x=c(0,0.9))
ggsave("plotcorr2.png",width=6,height=3)

In this particular context (where both variables are measured on the same scale), there’s also nothing wrong with plotting them both against the date axis:

library(reshape2)
library(plyr)
m1 <- rename(melt(dat,id.vars=c("words","dates","month")),
             c(variable="party"))

ggplot(m1,aes(dates,value,colour=party))+geom_line()+
    geom_text(aes(label=words),size=3)+
    expand_limits(x=as.Date(c("2009-12-15","2010-10-01")))
ggsave("plotcorr3.png",width=6,height=3)